It’s not natural for pregnancies that young.
At 12, a girl is not developed enough to bear a healthy baby. At 16-17, her body is fully grown.
100 years ago women were getting married in their teens all the time. It still happens in small towns. While black neighborhoods have preteens with babies. 500 years ago the average lifespan was 30. Girls in some ethnic groups aren’t ready until they are older, late teens. But in other groups they are starting at 12.
Say what you wish about the morality but the human race would never have gotten this far if our ancestors waited until they were 30 years old to start having children. This thing called adolescence is a resent phenomenon. Even as recently as 1900 in America the average lifespan was 47. Think if a girl waited to 25 she would be dead before her first grandchild. And likely she would not live long enough to raise all her kids.